MM All,
The purpose of circle for me has two meanings, as I cast two different types of circle. One type of circle I cast is circle for ritual working, and the other type is just a "mind" circle. I'll discuss the mind circle first.
I cast mind circles before meditation or visualization. The purpose of doing this is to calm my mind. It also gives me a sense of security, especially when doing visualization that involves the air element. I say this as I feel the circle protects me from astral traveling without meaning to as I do not like when this happens. The purpose of mind circles is also sometimes to help me overcome particularly scary nights. I mentioned in a previous post that I do not suffer from depression, but suffer from fears. At times when I am being overcome by all my fears, I cast a mind circle to focus my mind on something else, and to give me a sense of security. When I post my lesson on how to cast circle, I will post how I cast a mind circle. The other form of circle is of course the circle I cast for ritual. I classed the two circles differently as I cast them differently, and use them for different purposes. The purpose of circle is the place oneself outside of the mundane world. The processes, and habit, of casting a circle puts a person in the correct frame of mind for performing ritual. The circle also serves as a boundary to contain energy. Any energy raised for magickal working is contained by the boundary of the circle, and only released at the will of the practitioner.
Another purpose of circle not often used in Wiccan texts is the purpose of attuning a person to the universe. A circle has been seen as a sacred symbol in many cultures. A circle was drawn around Roman ambassadors in foreign countries to show he was safe from attack, and women in the Middle Ages traced a circle around a bed during labour to keep away bad spirits. This relates to some magickal practitioners using the circle as a security precaution.
One of the most famous circles in the world is Stonehenge. Stonehenge was built approximately 4,200 years ago, and served many purposes. It was a demarcation of a sacred area, a boundary for ritual, and a universal calendar. Stonehenge has been proved to be a precise measure of the phases of the moon, summer/winter solstice and many eclipses. It is obvious that this circle provided attunement to the universe through being able to see when certain events would happen, but also through the ritual that would have occurred within the circle.
Circles can be seen to represent many aspects of the universe. They can represent the planets, moons, life-death-rebirth cycle, the sun, and the ovum/seed of fruits or unity of self. For me then the purpose of circle is to remind me of all things a circle could represent in the universe, and to assist me in attuning to the universe. So there you have the two ways I think about casting circle, and the numerous purposes of each.
Sources used : Raymond Buckland's Complete Book of Witchcraft 101 Sacred Places on Earth (authors name can't be remembered at the moment, sorry)
Blessed Be Miw Sheri